Update README.rst (#8)

Will Myers · web-flow · commit 92bda72e15d7 · 2016-11-01T11:50:49.000-04:00
diff --git a/README.rst b/README.rst
@@ -114,6 +114,7 @@ Create
         'currency': 'USD',
         'order_id': '2007',
         'finalization_date': '2017-08-02T19:26:08Z',
+        'btn_ref': 'srctok-XXX',
     })
 
     print(response)
